FIELD OF THE INVENTION
The present invention relates to electrically conductive shaped articles of thermoplastic material and to their use.
Plastics which comprise conductive fillers, in particular steel fibers, are used for the production of components of high electrical conductivity and, if appropriate, additionally good mechanical properties.
The wide use of theme conductive plastics for the production of shaped articles is counteracted by the high material costs of the stainless steel filaments, which are about 20 to 30 times those of glass fibers.
Cheaper carbon black fillings are not capable of achieving conductivities which are required for significant electromagnetic shielding. In addition, the mechanical properties deteriorate because of the carbon black filling. Thus, test articles of various polyamides having a content of 30% by weight of carbon black show only half the tensile strength of the non-filled starting material.
The content of stainless steel filaments/steel filaments necessary to render a plastic electrically conductive can be modified only within certain limits. If the amount is no longer adequate to develop a conductivity network, the conductivity drops sharply.
In order to increase the conductivity, according to U.S. Pat. No. 4 973 514, metal fibers or metalized glass fibers can be incorporated into the plastic, advantageously with the addition of finely divided conductive fillers, such as carbon black, for the production of shaped articles. If non-conductive fibers furthermore are added, the electromagnetic shielding effect in intensified. The process mentioned is similar to the production of paper. To produce massive shaped articles, it is necessary for the conductive film initially obtained to be cut into chips and for the chips to be processed by injection molding. The process described requires effort and is expensive. The short length and poor distribution of the fibers in the shaped article leads to only weak shielding efficiencies.
In the period from 1989 to 1993, mixtures of fiber-reinforced pellets (length 1 to 2 cm) of thermoplastic material which were reinforced on the one hand by steel fibers (about 4 to 10% by weight) and on the other hand by glass fibers (20 to 60% by weight) were sold (polymer composites, Inc., USA) under the name .RTM.CELSTRAN S. However, no pellets which were reinforced simultaneously by steel fibers and glass fibers were sold.
There was therefore the object of producing shaped articles which comprise steel fibers, can easily be produced, and in particular have a high conductivity and therefore good shielding from electromagnetic waves.
The present invention achieves this object.